In Settings, search for “System UI”.
Tap General Notifications
Select “silent and minimized”
Didn’t work so far. Still sounding something. Maybe need to find a different entry.
Tap allow interruptions. Disable it.
Both networkdiagnostic and system UI show in the list of appa, when you enable “show system apps”.
Next try is the “phone” app. Unfortunately there are 4 apps called phone. Tried to silence all, still at least vibrating, so still no winner.